Sensitivity of resolved mountain drag to model resolution for MAP case-studies

Seven mountain wave case-studies from the Mesoscale Alpine Programme (MAP) have been used to investigate the behaviour of the resolved Alpine pressure drag with model horizontal resolution. Three independent modelling systems show that the magnitude of the resolved surface pressure drag increases monotonically as the horizontal model resolution is increased from 125 km down to a few kilometres. A substantial proportion of the drag will still need to be parametrized in the next generation of mesoscale models, with horizontal resolutions of 3 or 4 km. This is particularly true for the most important drag cases, when the jet stream directly above the mountains produces larger wind speeds near mountain crest level, resulting in much larger total Alpine drags.